Processes for recovering rubber from natural rubber latex

ABSTRACT

Processes for recovering natural rubber from an aqueous natural rubber latex that contains extractables and one or more additives and that is essentially free of lignocellulosic plant material are described. Natural rubber in the latex is separated from the water. Preferably, the amount of non-rubber extractables in the natural rubber is reduced by one or more extraction steps yielding a dried natural rubber.

PRIORITY

This application claims priority to U.S. provisional application Ser.No. 61/044649, filed Apr. 14, 2008.

FIELD

The present application relates to processes for the recovery of naturalrubber from natural rubber latex, and particularly from natural rubberlatex containing water, extractables, and one or more additives, andthat is essentially free of lignocellulosic plant material.

BACKGROUND

Natural rubber may be obtained from a number of plants, including Heveatrees (Hevea brasiliensis) and Guayule shrubs (Parthenium argentatum),among others. Processes for recovering or harvesting natural rubber fromthese sources are known in the art. For example, Hevea trees are usuallytapped such that the Hevea sap (containing natural rubber) is harvestedand contacted with an organic acid to coagulate the natural rubber, andthereafter concentrated by evaporation or centrifugation. Alternatively,Guayule shrubs are usually ground up or otherwise masticated to releasesap (containing natural rubber) from the cell walls of the shrub, andthe pulp thereafter contacted with solvent or aqueous acids in order tocoagulate the natural rubber, and thereafter concentrated. Otherprocesses for recovering or harvesting natural rubber from naturalsources are known.

SUMMARY

Disclosed herein are processes for recovering rubber from an aqueousnatural rubber latex that contains natural rubber, water, extractables,and one or more additives, and is essentially free of lignocellulosicplant material. In one embodiment, the process comprises the steps of:contacting the latex with a separating solvent composition comprising atleast one hydrocarbon solvent and at least one organic polar solventwhereby at least 75% of the natural rubber is separated from the waterand a solution having a viscous rubber phase containing natural rubberand a reduced amount of extractables is formed; isolating the viscousrubber phase; and drying the viscous rubber phase to create a resultantnatural rubber wherein the amount of extractables is 5% by weight orless.

Another embodiment includes a process comprising the steps of:contacting the aqueous natural rubber latex with a first treatmentsolution whereby at least 75% of the natural rubber coagulates and amixture containing coagulated natural rubber, water, and extractables isformed; collecting the coagulated natural rubber; contacting thecoagulated natural rubber with a second treatment solution comprising atleast one organic polar solvent in order to preferentially solvate theextractables; and drying the coagulated natural rubber to create aresultant natural rubber wherein the amount of extractables is 5% byweight or less.

Yet another embodiment includes a process comprising the steps of:freezing the aqueous natural rubber latex, whereby at least 75% of thenatural rubber coagulates and a mixture containing coagulated naturalrubber, water, and extractables is formed; thawing the mixture;collecting the coagulated natural rubber; contacting the coagulatednatural rubber with a treatment solution comprising at least one organicpolar solvent in order to preferentially solvate the extractables; anddrying the coagulated natural rubber to create a resultant naturalrubber wherein the amount of extractables is 5% by weight or less.

DETAILED DESCRIPTION

Disclosed herein are processes for recovering rubber from an aqueousnatural rubber latex that contains natural rubber, water, extractables,and one or more additives, and is essentially free of lignocellulosicplant material.

In one embodiment, the process comprises the steps of: contacting thelatex with a separating solvent composition comprising at least onehydrocarbon solvent and at least one organic polar solvent whereby atleast 75% of the natural rubber is separated from the water and asolution having a viscous rubber phase containing natural rubber and areduced amount of extractables is formed; isolating the viscous rubberphase; and drying the viscous rubber phase to create a resultant naturalrubber wherein the amount of extractables is 5% by weight or less. Whenthe natural rubber is separated from the water of the aqueous naturalrubber latex it is contemplated that the natural rubber coagulates toform a viscous rubber phase. The use of the term viscous rubber phaseherein is meant to include a swollen gel and a viscous liquid containingrubber.

Generally, any hydrocarbon solvent may be used in the separating solventcomposition provided it preferentially solvates natural rubber in anatural rubber latex. For example, hydrocarbon solvents suitable for useinclude but are not limited to the following: alkanes having from 4 to 9carbon atoms such as heptane, nonane, pentane, and preferably hexane;cycloalkanes having from 5 to 10 carbon atoms such as cyclohexane,cyclopentane, and the like; aromatics or alkyl substituted aromaticshaving from 6 to 12 carbon atoms such as benzene, xylene, toluene, andthe like. Mixtures of two or more hydrocarbons may also be used.Furthermore, any organic polar solvent may be used in the separatingsolvent composition provided it preferentially solvates a portion ofnon-rubber extractables in a natural rubber latex and acts to coagulatenatural rubber. Broadly stated, extractables are any naturally occurringnon-rubber chemical entity present in rubber, including but not limitedto resins (such as terpenes), fatty acids, proteins, and inorganicmaterials. For example, organic polar solvents suitable for use includebut are not limited to the following: alcohols having from 1 to 8 carbonatoms such as ethanol, isopropanol, methanol, and the like; and ketoneshaving from 3 to 8 carbon atoms such as acetone, methyl ethyl ketone,and the like. Mixtures of two or more organic polar solvents may also beused.

Normally, organic polar solvents act to coagulate the natural rubber inthe latex, whereas hydrocarbons act to solvate the natural rubber. Inthis way, the ratio of organic polar solvent to hydrocarbon solvent inthe separating solvent composition influences the state or phase of thenatural rubber after contact of the latex with the separating solventcomposition. Generally, as the relative fraction of organic polarsolvent to hydrocarbon solvent is increased, the natural rubberincreasingly coagulates to form a semi-solid or solid mass. In contrast,as the fraction of hydrocarbon solvent is increased, the natural rubberincreasingly forms a swollen gel or viscous liquid, as the naturalrubber is increasingly solvated by the hydrocarbon. It is generallypreferred to adjust the ratio such that a swollen gel or viscous liquidphase of the natural rubber is obtained in the latex after it has beencontacted by the separating solvent composition. Preferably, the atleast one hydrocarbon solvent is present in an amount between 5% and 60%by weight and the at least one organic polar solvent is present in anamount between 95% and 40% by weight, based on the total weight of theseparating solvent composition. Optionally, the at least one hydrocarbonsolvent is present in an amount between 5% and 25% by weight and the atleast one organic polar solvent is present in an amount between 75% and95% by weight, based on the total weight of the separating solventcomposition.

The amount of separating solvent composition utilized to contact theaqueous natural rubber latex may vary and is generally an amountsufficient to separate at least 75% of the natural rubber from thewater, and preferably to reduce the extractables content of the naturalrubber that is recovered from the aqueous natural rubber latex. Forexample, in one embodiment the weight ratio of separating solventcomposition to latex is 100 to 1, respectively. In another embodiment,the ratio is 20 to 1. In yet another embodiment, the ratio is 10 to 1.Moreover, in certain embodiments, multiple steps of contacting theaqueous natural rubber latex with a separating solvent composition maybe utilized in order to further reduce the extractables content of thenatural rubber that is recovered from the aqueous natural rubber latex.

Generally, any method of contacting the aqueous natural rubber latexwith the separating solvent composition known to those skilled in theart may be used, including but not limited to any continuouscountercurrent liquid-liquid contacting system (such as mixer-settlerand column arrangements) and any batch-type system.

After the aqueous natural rubber latex has been contacted with theseparating solvent solution and a solution having a viscous rubber phasecontaining natural rubber and a reduced amount of extractables has beenformed, the viscous rubber phase is isolated from one or more otherphases that may have been formed using any suitable method known tothose skilled in the art, including but not limited to decanting the oneor more other phases away from the viscous rubber phase. Once isolated,the viscous rubber phase is dried to create a resultant natural rubberthat contains an amount of extractables of 5% by weight or less. Incertain embodiments, the amount of extractables in the dried resultantnatural rubber is 1% by weight or less. Generally, any method of dryingthe viscous rubber phase known to those skilled in the art may be used,including but not limited to the application of heat and vacuum.Generally, the resultant natural rubber is dried until it contains lessthan 1% solvent by weight. In certain embodiments, the amount of anysolvent in the dried resultant natural rubber is less than 0.1% byweight.

A resultant natural rubber that is solid and that may be ready forfurther processing, such as vulcanizing, is obtained upon drying theviscous phase.

Optionally, prior to drying the viscous rubber phase, it is contactedwith at least one extracting solvent composition comprising at least oneorganic polar solvent. Generally, any organic polar solvent may be usedprovided it preferentially solvates non-rubber extractables in naturalrubber. As discussed previously, extractables are any naturallyoccurring non-rubber chemical entity present in rubber, including butnot limited to resins (such as terpenes), fatty acids, proteins, andinorganic materials. Organic polar solvents suitable for use include butare not limited to the following: alcohols having from 1 to 8 carbonatoms such as ethanol, isopropanol, methanol, and the like; and ketoneshaving from 3 to 8 carbon atoms such as acetone, methyl ethyl ketone,and the like. Mixtures of two or more organic polar solvents may also beused. In a particular embodiment, the extracting solvent compositioncomprises acetone.

Since organic polar solvents tend to preferentially solvate non-rubberextractables that are present in natural rubber, an extracting solventcomposition that contacts natural rubber solvates, or extracts, aportion of the non-rubber extractables. A number of methods exist andare known to those skilled in the art for contacting the viscous rubberphase with the extracting solvent composition, any of which may be usedherein, including but not limited to any continuous countercurrentextraction system (such as mixer-settler and column arrangements) andany batch-type system.

Natural rubber (“rubber”) comprises cis-polyisoprene and, as usedherein, also includes trans-polyisoprene and mixtures of the two. Anumber of various plants produce natural rubber in the form of sap thatcontains the natural rubber in aqueous suspension. In certain plants,such as Hevea brasiliensis (Hevea trees), Ficus elastica (India rubbertree), and Cryptostegia grandiflora (Madagascar rubbervine), therubber-bearing sap flows freely and is recovered simply by tapping theplant. In other plants (“non-Hevea plants”) the rubber-bearing sap isnot as accessible, since the sap is stored in individual cells containedwithin the roots or stems, which cells must be broken down by physicalor other means. Such plants include but are not limited to Partheniumargentatum (Guayule shrub), Taraxacum Kok-Saghyz (Russian dandelion),Euphorbia lathyris (gopher plant), Parthenium incanum (mariola),Chrysothamnus nauseosus (rabbitbrush), Pedilanthus macrocarpus(candililla), Asclepias syriaca, speciosa, subulata, et al (milkweeds),Solidago altissima, graminifolia rigida, et al (goldenrods), Cacaliaatripilicifolia (pale Indian plantain), Pycnanthemum incanum (mountainmint), Teucreum canadense (American germander) and Campanula Americana(tall bellflower). Many other plants which produce rubber andrubber-like hydrocarbons are known, particularly among the Compositae,Euphorbiaceae, Campanulaceae, Labiatae, and Moracea families. Mixturesmay be used.

In certain embodiments, the processes for recovering rubber from anaqueous natural rubber latex that are disclosed herein are performedutilizing a rubber-bearing sap that has been harvested or recovered froma non-Hevea plant. In other words, in those embodiments therubber-bearing sap has not been harvested or recovered from a Heveatree, an India rubber tree or a Madagascar rubbervine. Preferably, therubber-bearing sap has been harvested or recovered from a Guayule shrub.

The aqueous natural rubber latex that is utilized in the processesdisclosed herein may have resulted from contacting rubber-bearing sapwith various solvents and other additives and/or otherwise processingrubber-bearing sap that has been harvested or recovered from a naturalsource. Additives that are often utilized include, but are not limitedto, various solvents, surfactants, and surface active agents. Therubber-bearing sap typically contains non-rubber extractables which varyby type and amount depending on source. Lignocellulosic plant material(material that comprises the structural cells of woody plants) has alsotypically been removed by physical methods, such as filtration so thatthe aqueous natural rubber latex utilized in the processes disclosedherein is essentially free of lignocellulosic plant material. Byessentially free of lignocellulosic plant material is meant that themajority of such material has been removed, preferably at least 95% ofsuch material has been removed, and even more preferably at least 99% ofsuch material has been removed.

Rubber is typically cross-linked, or vulcanized, in order to make usefulproducts. While not wishing to be bound by any theory, it is believedthat extractables may adversely affect the vulcanization process and/orthe physical properties of vulcanized products. In the processesdescribed herein, the natural rubber (either coagulated or in a viscousphase) may be contacted any number of times using an extracting solventcomposition in order to reduce the amount of extractables contained inthe rubber, and thereafter dried to create a resultant natural rubberwith a reduced amount of extractables. In one embodiment, theextractables content of the resultant natural rubber is reduced to anamount of 5% by weight or less (based on the weight of the resultantnatural rubber). In yet another embodiment, the extractables content inthe resultant natural rubber is 2% by weight or less. In yet anotherembodiment, the extractables content in the resultant natural rubber is1% by weight or less.

Another embodiment of the process for recovering rubber from an aqueousnatural rubber latex that contains natural rubber, water, extractables,and one or more additives, and is essentially free of lignocellulosicplant material comprises the steps of: contacting the latex with a firsttreatment solution whereby at least 75% of the natural rubber coagulatesand a mixture containing coagulated natural rubber, water, andextractables is formed; collecting the coagulated natural rubber;contacting the coagulated natural rubber with a second treatmentsolution comprising at least one organic polar solvent in order topreferentially solvate the extractables; and drying the coagulatednatural rubber to create a resultant natural rubber wherein the amountof extractables is 5% by weight or less.

Various solvents and chemical, in various combinations, may be utilizedfor the first treatment solution. More specifically, the first treatmentsolution may comprise any solution capable of coagulating at least 75%of the natural rubber in aqueous natural rubber latex. In oneembodiment, the first treatment solution comprises an organic acid, amineral acid, a salt of either and combinations thereof. An organic acidis generally any acid with a carbon-atom backbone, often containing acarboxyl group, including but not limited to any carboxylic acid. Amineral acid is generally an acid derived from one or more inorganiccompounds that does not contain any carbon atoms, including but notlimited to sulfuric acid, hydrochloric acid and nitric acid. Preferably,the first treatment solution comprises acetic acid, formic acid,sulfuric acid, aqueous aluminum sulfate, aqueous calcium chloride, andmixtures thereof. In one embodiment, the first treatment solutioncomprises an organic acid, a mineral acid, a salt of either, andcombinations thereof, and the amount of extractables in the resultantnatural rubber is 5% by weight or less, preferably 2% by weight or lessand even more preferably 1% by weight or less.

In another embodiment, the first treatment solution comprises at leastone organic polar solvent. Organic polar solvents suitable for useinclude but are not limited to the following: alcohols having from 1 to8 carbon atoms such as ethanol, isopropanol, methanol, and the like; andketones having from 3 to 8 carbon atoms such as acetone, methyl ethylketone, and the like. Mixtures of two or more organic polar solvents mayalso be used. In one embodiment, the first treatment solution comprisesat least one organic polar solvent and the amount of extractables in theresultant natural rubber is 5% by weight or less, preferably 2% byweight or less and even more preferably 1% by weight or less.

Generally any method of contacting the aqueous natural rubber latex withthe first treatment solution known to those skilled in the art may beused, including but not limited to any continuous countercurrentliquid-liquid contacting system (such as mixer-settler and columnarrangements) and any batch-type system.

After the aqueous natural rubber latex is contacted with a firsttreatment solution and a mixture containing coagulated natural rubber,water and extractables is formed, the coagulated natural rubber iscollected. The coagulated natural rubber is collected by any suitablemethod, including but not limited to centrifugation, filtration, andphase separation. For example, in certain embodiments, a viscous-typephase containing the natural rubber may result along with one or moreliquid phases. The viscous rubber phase may be separated by variousmethods within the knowledge of one of skill in the art, including, butnot limited to, decanting the liquid phase or phases away from theviscous rubber phase. Optionally, the collected coagulated naturalrubber may be rinsed with one or more portions of water.

After the coagulated natural rubber has been collected, it is contactedwith a second treatment solution comprising at least one organic polarsolvent. The second treatment solution may generally comprise anyorganic polar solvent provided it preferentially solvates non-rubberextractables in natural rubber. Organic polar solvents suitable for useinclude but are not limited to the following: alcohols having from 1 to8 carbon atoms such as ethanol, isopropanol, methanol, and the like; andketones having from 3 to 8 carbon atoms such as acetone, methyl ethylketone, and the like. Mixtures of two or more organic polar solvents mayalso be used. The second treatment solution may be the same as ordifferent from the first treatment solution. The second treatmentsolution may also comprise, in addition to the at least one organicpolar solvent, at least one hydrocarbon solvent.

Prior to contacting the coagulated natural rubber with the secondtreatment solution, it may be advantageous in some embodiments to reducethe size of the coagulated rubber by, for example, shredding or tearingthe rubber into pieces. Generally, any method of contacting thecoagulated natural rubber with the second treatment solution known tothose skilled in the art may be used, including but not limited to anycontinuous countercurrent liquid-liquid contacting system (such asmixer-settler and column arrangements) and any batch-type system.Additionally, it is explicitly contemplated that in certain embodimentsthe coagulated natural rubber may be contacted with the second treatmentsolution prior to collecting the coagulated natural rubber.

After the coagulated natural rubber is contacted with a second treatmentsolution, it is dried to create a resultant natural rubber with anamount of extractables of 5% by weight or less (based on the weight ofthe dried resultant natural rubber). Generally, any method of drying thecoagulated natural rubber known to those skilled in the art may be used,including but not limited to the application of heat and vacuum.Generally, the resultant natural rubber is dried until it contains lessthan 1% solvent by weight. In certain embodiments, the amount of anysolvent in the dried resultant natural rubber is less than 0.1% byweight.

The resultant natural rubber that is created when the coagulated naturalrubber is dried contains an amount of extractables of 5% by weight orless, based on the weight of the dried natural rubber. In certainembodiments, the amount of extractables in the dried resultant naturalrubber is 2% by weight or less or 1% by weight or less.

Optionally, before the coagulated natural rubber is dried, in certainembodiments the mixture that results from contacting the coagulatednatural latex with a second treatment may be separated from one or moreother phases. For example, in certain embodiments, a viscous-type phasecontaining the natural rubber may result along with one or more liquidphases. The viscous rubber phase may be separated by various methodswithin the knowledge of one of skill in the art, including, but notlimited to, decanting the liquid phase or phases away from the viscousrubber phase.

The presence of an amount of carbon black in the coagulated naturalrubber can increase the amount of extractables that are solvated, orextracted, from the natural rubber by an extracting solvent (e.g., aseparating solvent composition, a first treatment solution, or a secondtreatment solution), such as an organic polar solvent. While not wishingto be bound by a particular theory, it is believed that the presence ofcarbon black increases porosity in the coagulated natural rubber, andthereby allows an extracting solvent to penetrate the coagulated naturalrubber and contact a relatively large surface area of the rubber.

In one embodiment of the processes described herein, carbon black isoptionally added to the aqueous natural rubber latex prior to contactingthe latex with a first treatment solution. The carbon black is added inan amount between 0.5% and 6% by weight based on the weight of theresultant dried natural rubber. Generally, the carbon black may be addedto the aqueous natural rubber latex by any method known to those skilledin the art, including but not limited to creating an aqueous dispersionor slurry of carbon black and subsequently combining the dispersion orslurry with the aqueous natural rubber latex.

In another embodiment, carbon black is optionally added to thecoagulated natural rubber prior to the contacting the coagulated naturalrubber with a second treatment solution. The carbon black is added in anamount between 0.5% and 6% by weight based on the weight of theresultant dried natural rubber. Generally, the carbon black may be addedto the coagulated natural rubber by any method known to those skilled inthe art, including but not limited to creating an aqueous dispersion orslurry of carbon black and subsequently combining the dispersion orslurry with the coagulated natural rubber accompanied by kneading,mixing, or other physical mechanism to force the dispersion or slurryinto the coagulated natural rubber.

In a particular embodiment, carbon black is added to either the aqueousnatural rubber latex or the coagulated natural rubber (or both), and thefirst and second treatment solutions consist essentially of acetone.

Yet another embodiment of the process for recovering rubber from anaqueous natural rubber latex that contains natural rubber, water,extractables, and one or more additives, and is essentially free oflignocellulosic plant material comprises the steps of: freezing thelatex, whereby at least 75% of the natural rubber coagulates and amixture containing coagulated natural rubber, water, and extractables isformed; thawing the mixture; collecting the coagulated natural rubber;contacting the coagulated natural rubber with an extraction solutioncomprising at least one organic polar solvent in order to preferentiallysolvate the extractables; and drying the coagulated natural rubber tocreate a resultant natural rubber wherein the amount of extractables is5% by weight or less.

In a first step, the aqueous natural rubber latex is frozen to create amixture containing natural rubber, water and extractables. In general,freezing an aqueous natural rubber latex effects coagulation of thesuspended natural rubber, and typically any method for freezing anaqueous solution is suitable for the processes described herein. Suchfreezing methods include but are not limited to: providing asub-freezing atmosphere to the latex and/or providing cooling elements(such as fins, plates, scrapped tube walls or rolls) to the latex.Freezing the aqueous natural rubber proceeds until it is completelyfrozen or until at least 75% of the natural rubber has been coagulated,upon which a mixture containing coagulated natural rubber, water(completely or partially frozen), and extractables is obtained. It isnoted that at this point in the process the extractables may be presentin both the coagulated natural rubber and in the water.

After the aqueous natural rubber latex has been frozen, completely orpartially, the frozen mixture is thawed. Generally, thawing may beaccomplished by any suitable method known to one of ordinary skill inthe art, including but not limited to exposing the frozen mixture to theambient atmosphere, contacting the frozen mixture with a heating elementor elements, and placing the frozen mixture in an oven (with or withoutvacuum).

After the frozen mixture is thawed, the coagulated natural rubber iscollected. Generally, any suitable method for collecting coagulatednatural rubber in an aqueous medium known to one of ordinary skill inthe art may be used, including but not limited to centrifugation,flotation, and filtration.

After the coagulated natural rubber has been collected, it is contactedwith an extraction solution comprising at least one organic polarsolvent. The extraction solution may generally comprise any organicpolar solvent provided it preferentially solvates non-rubberextractables in natural rubber. Organic polar solvents suitable for useinclude but are not limited to the following: alcohols having from 1 to8 carbon atoms such as ethanol, isopropanol, methanol, and the like; andketones having from 3 to 8 carbon atoms such as acetone, methyl ethylketone, and the like. Mixtures of two or more organic polar solvents mayalso be used. The extraction solution may also comprise, in addition tothe at least one organic polar solvent, at least one hydrocarbonsolvent.

Prior to contacting the coagulated natural rubber with the extractionsolution, it may be advantageous in some embodiments to reduce the sizeof the coagulated rubber by, for example, shredding or tearing therubber into pieces. Generally, any method of contacting the coagulatednatural rubber with the extraction solution known to those skilled inthe art may be used, including but not limited to any continuouscountercurrent liquid-liquid contacting system (such as mixer-settlerand column arrangements) and any batch-type system. Additionally, it isexplicitly contemplated that in certain embodiments the coagulatednatural rubber may be contacted with the extraction solution prior tocollecting the coagulated natural rubber.

After the coagulated natural rubber is contacted with the extractionsolution, it is dried to create a resultant natural rubber with anamount of extractables of 5% by weight or less (based on the weight ofthe dried resultant natural rubber). Generally, any method of drying thecoagulated natural rubber known to those skilled in the art may be used,including but not limited to the application of heat and vacuum.Generally, the resultant natural rubber is dried until it contains lessthan 1% solvent by weight. In certain embodiments, the amount of anysolvent in the dried resultant natural rubber is less than 0.1% byweight.

The resultant natural rubber that is created when the coagulated naturalrubber is dried contains an amount of extractables of 5% by weight orless, based on the weight of the dried natural rubber. In certainembodiments, the amount of extractables in the dried resultant naturalrubber is 2% by weight or less or 1% by weight or less.

Optionally, before the coagulated natural rubber is dried, in certainembodiments the mixture that results from contacting the coagulatednatural latex with the extraction solution may be separated from one ormore other phases. For example, in certain embodiments, a viscous-typephase containing the natural rubber may result along with one or moreliquid phases. The viscous rubber phase may be separated by variousmethods within the knowledge of one of skill in the art, including, butnot limited to, decanting the liquid phase or phases away from theviscous rubber phase.

The presence of an amount of carbon black in the coagulated naturalrubber can increase the amount of extractables that are solvated, orextracted, from the natural rubber by an extracting solvent, such as anorganic polar solvent. In one embodiment of the processes describedherein, carbon black is optionally added to the aqueous natural rubberlatex prior to freezing the latex. The carbon black is added in anamount between 0.5% and 6% by weight based on the weight of theresultant dried natural rubber. Generally, the carbon black may be addedto the aqueous natural rubber latex by any method known to those skilledin the art, including but not limited to creating an aqueous dispersionor slurry of carbon black and subsequently combining the dispersion orslurry with the aqueous natural rubber latex.

In another embodiment, carbon black is optionally added to thecoagulated natural rubber prior to contacting the coagulated naturalrubber with the extraction solution. The carbon black is added in anamount between 0.5% and 6% by weight based on the weight of theresultant dried natural rubber. Generally, the carbon black may be addedto the coagulated natural rubber by any method known to those skilled inthe art, including but not limited to creating an aqueous dispersion orslurry of carbon black and subsequently combining the dispersion orslurry with the coagulated natural rubber accompanied by kneading,mixing, or other physical mechanism to force the dispersion or slurryinto the coagulated natural rubber. In a particular embodiment, carbonblack is added to either the aqueous natural rubber latex or thecoagulated natural rubber (or both), and the extraction solution consistessentially of acetone.

The invention will be more readily understood by reference to thefollowing examples. There are, of course, many other embodiments orillustrations which will become apparent to one skilled in the art, andit will accordingly be recognized that these examples are given for thepurpose of illustration only, and are not to be construed as limitingthe scope of this invention in any way.

EXAMPLES

The following test procedures were used in carrying out several of theexamples, as indicated below.

-   -   (a) Determination of the amount of extractables—ASTM D297,        Section 18, modified to allow use of a standard Soxhlet        extraction apparatus, with acetone.    -   (b) Micro Dumbbell Testing—ASTM D412.    -   (c) Standard natural rubber test formulation—see Table 1

TABLE 1 Ingredient Parts per 100 rubber Rubber 100 N339 Carbon Black 50Stearic Acid 2 Antioxidant (6PPD)¹ 1 Zinc Oxide 3 Accelerator² 0.8Sulfur 1.3 Total 158.1 ¹-Antioxidant is SANTOFLEX 6PPD which isN-(1,3-dimethylbuty1)-N′-phenyl-p- phenylenediamine, produced and soldby FLEXSYS, Akron, Ohio. ²-Accelerator is N-tert-butyl-2-benzothiazolesulfenamide (TBBS).

The preparation of the natural rubber (NR) formulations involves mixingthe components shown in the table by use of a Brabender Mixer, and thenmolding to sheets having a thickness of about 2 mm, followed by pressingcure at 145° C. for 30 minutes.

Example 1

A 149.03 gram sample of an aqueous natural rubber latex marketed byYulex Corp. and having about 48.9% total solids from guayule shrub(about 40.7% purified rubber) was mixed with 3 liters of a 50/50(volume) composition of acetone and mixed hexane isomers for about 30minutes. This resulted in the formation of one viscous rubber phase andtwo liquid phases.

The liquid phases were decanted. The solid phase was squeezed to removetrapped pockets of liquid in the wet rubber. The squeezed rubber was cutinto small pieces, and mixed with a further 3 liters of a freshcomposition comprising 50/50 (volume) of acetone and mixed hexaneisomers, for an additional 30 minutes. The resultant liquid wasdecanted, and the resultant solid rubber was squeezed. The squeezedsolid rubber was vacuum dried to remove all solvent present. The dryrubber weight was 60.06 grams. By calculation, it is determined that thecoagulation of the rubber latex with the 50/50 (volume) acetone/hexanecomposition, yielded 60.06 grams of rubber, which indicates that 12.82grams of extractables were removed. The calculation is made as follows:

0.489×149.03=72.88 grams solids in latex sample;

72.88 grams solids−60.06 grams rubber=12.82 grams of extractablesremoved.

In order to determine the amount of extractables remaining in the driedrubber, a small sample of the dried rubber was subjected to a Soxhletextraction, with acetone. As a result, there was obtained an additional0.90 weight % extractables, based on the weight of the dried rubber,indicating that the dried rubber contained 0.90 weight % extractables.

The dry rubber was dissolved in hexane to form a uniform solution. Onepart per 100 parts rubber (by weight) of Santoflex 134 antioxidant,produced and sold by Flexsys Rubber Chemicals, Akron, Ohio was added,and kneaded into the rubber solution. The solvent was then removed byvacuum drying.

The resultant dried and stabilized rubber, identified as Sample 1, wasthen placed in a 60° C. forced air oven for storage stabilityevaluation. Mooney viscosities as shown in Table 2A below were measuredbefore and after aging in the oven.

TABLE 2A Sample 1 Before Aging After 7 Days ML (1 + 4) 95.1 94.7

The resultant dried and stabilized rubber product was further evaluatedby compounding the dry rubber product in the standard all-natural rubbertest recipe described herein. Test data for the compounded dried rubberproduct of Example 1, identified as Sample 1A, is listed with data forcommercially available Hevea natural rubber, identified as Hevea #1, inthe following Table 2B for the purpose of comparison.

TABLE 2B Micro Dumbbell Tensile @23 C Sample lA Hevea #1 Max Stress(MPA) 32.5 31.5 100% (MPA) 3.0 3.3 200% (MPA) 9.8 10.2 300% (MPA) 18.019.6 Brk Strain % 497 448.9 Toughness 71.7 61.6 Micro Dumbbell Tensile@100 C Sample lA Hevea #1 Max Stress (MPA) 21.2 21.8 100% (MPA) 2.5 2.6200% (MPA) 6.3 6.5 300% (MPA) 11.0 11.7 Brk Strain % 662 513.6 Toughness76.5 51.4

Also, for comparative purposes, gel permeation chromatography (GPC) wasperformed on Sample 1, with the following results: Mn=464,141;Mw=1,288,439; Mw/Mn=2.78.

From the data in Table 2B, it is observed that the rubber productobtained in Example 1 compares very favorably in performance to thecommercially available Hevea natural rubber (Hevea #1).

Example 2

Example 2 utilizes formic acid in coagulation of the natural rubberlatex, rather than the composition of hydrocarbon solvent and organicpolar solvent, as in Example 1.

A 200 gram sample of an aqueous natural rubber latex marketed by YulexCorp. and having about 50% total solids (about 41% purified rubber) wasdiluted by adding 90 grams of distilled water, resulting in a mixturehaving a pH of 8.0. To the natural rubber latex was added 25 grams of a5% formic acid solution in water, and the resulting mixture was stirredfor about 1 hour, to complete the coagulation of the rubber. At thispoint, the pH was between 5 and 6. The rubber sample was then vacuumdried, to remove residual water.

In order to determine the amount of extractables remaining in the driedrubber, a small sample of the dried rubber product prepared in thisexample was then subjected to a Soxhlet extraction utilizing acetone.The Soxhlet extraction revealed that the dried rubber contained 11% ofacetone extractables, indicating that the dried polymer contained 11weight % extractables. This demonstrates that the process of Example 2is not as effective as the process of Example 1 in removing or reducingacetone extractable impurities from the dried rubber product.

When evaluated in the standard rubber test recipe, stated above, itbecomes clear that the degree to which acetone extractables are removedfrom the dried rubber product obtained from the natural rubber latex isan important factor in producing a product having properties comparableto Hevea natural rubber. In Table 3 below, the dried rubber product ofExample 2 is identified as Sample 2A, and the Hevea natural rubber isidentified as Hevea #1.

TABLE 3 Micro Dumbbell Tensile @23 C Sample 2A Hevea #1 Max Stress (MPA)17.1 31.5 100% (MPA) 1.3 3.3 200% (MPA) 2.8 10.2 300% (MPA) 5.3 19.6 BrkStrain % 679 448.9 Toughness 49.3 61.6 Micro Dumbbell Tensile @100 CSample 2A Hevea #1 Max Stress (MPA) 13.4 21.8 100% (MPA) 1.1 2.6 200%(MPA) 2.1 6.5 300% (MPA) 3.4 11.7 Brk Strain % 929 513.6 Toughness 56.951.4

Also, for comparative purposes, GPC was performed on Sample 2A prior tocompounding in the standard test recipe, with the following results:Mn=372,718; Mw=876,516; Mw/Mn=2.35.

From the above data, it is evident that the values obtained for thenatural rubber product of this example are, generally, lower than thevalues obtained for Hevea natural rubber, and it would be difficult toconclude that the two samples, Sample 2 and Hevea #1, are comparable.

Example 3

A 67.6 gram sample of an aqueous natural rubber latex marketed by YulexCorp. and having about 48.9% total solids from guayule shrub was dilutedby adding distilled water to a total weight of 338 grams, for anapproximate total solids content of 10%.

With strong agitation, the diluted latex was added slowly to a mixtureof 203.2 grams of hexane and 812.8 grams of acetone. When agitation wasstopped, the mixture separated within 10 minutes to form three distinctphases. After decanting the very fluid upper and lower liquid phases,the middle phase (a viscous phase), was recovered.

The viscous phase was then mixed with strong agitation with 457 grams ofhexane and 457 grams of acetone. After agitation, the mixture separatedquickly into two phases. The upper, very fluid phase was decanted, andthe lower, viscous phase was vacuum dried to constant weight.

The final weight of dry product, identified as Sample 3, weighed 23.66grams. After allowing for the weights of the various samples removedduring the process, the total weight of rubber in the original latex wasfound to be 28.80 grams, indicating a purified rubber content in theoriginal, undiluted latex of 42.6%.

In order to determine the amount of extractable in the dried finalproduct, a sample of the final rubber was extracted by the Soxhletprocedure with acetone, and found to contain 2.31 weight % acetoneextractables, indicating that the final rubber contained 2.31 weight %extractables.

A second small sample of the final rubber was stabilized withapproximately 1 part Santoflex 134 per 100 parts rubber, was analyzed byGPC and found to have the following parameters: Mn=482,888;Mw=1,211,165; Mw/Mn=2.51.

By comparison with the GPC results and % acetone extractables for therubber prepared in Example 1, it is reasonable to predict favorablecompounded tensile strength results compared to the Hevea #1 control.

Example 4

A 12.5 gram sample of the same aqueous natural rubber latex as utilizedin Example 3 was diluted with distilled water to a total weight of 625grams, for an approximate total solids content of 1%.

With strong agitation, the diluted latex was added slowly to a mixtureof 165.25 grams of hexane and 1652.65 grams of acetone. When agitationwas stopped, the mixture separated within 10 minutes to form threedistinct phases. After decanting the very fluid upper and lower phases,the middle phase (a viscous liquid) was recovered.

The viscous phase was then mixed with strong agitation with 90.63 gramsof hexane and 90.63 grams of acetone. After agitation, the mixtureseparated quickly into two phases. The upper, very fluid phase wasdecanted, and the lower, viscous phase was vacuum dried to constantweight.

The final weight of dry product weighed 4.93 grams, indicating apurified rubber content in the original, undiluted latex of 39.4%.

In order to determine the amount of extractables in the dried finalproduct, a sample of the final rubber was extracted by the Soxhletextraction procedure with acetone, and was found to contain 1.52 weight% acetone extractables, indicating that the dried polymer contained 1.52weight % extractables.

A second sample of the final rubber was stabilized with approximately 1part of Santoflex 134 per 100 parts rubber, and was analyzed by GPC andfound to have the following parameters: Mn=445,776; Mw=1,258,677;Mw/Mn=2.82.

By comparison with the GPC results and % acetone extractables for therubber prepared in Example 1, it is reasonable to predict favorablecompounded tensile strength results compared to the Hevea #1 control.

Example 5

A slurry of N339P carbon black, marketed by Columbian Chemicals ofProctor, W. Va. was formed by mixing 10.5 grams carbon black with anapproximately equal volume of distilled water, several grams of 40%potassium oleate paste, and several cc's of 5% aqueous KOH to raise thepH to approximately 8. The mixture was stirred until a thick slurry wasformed, free of lumps.

The thick slurry was added to 166.67 grams of an aqueous natural rubberlatex marketed by Yulex Corp. and having about 48.9% total solids fromguayule shrub (about 40.7% purified rubber) and stirred to blend. Nosettling of the carbon black and no coagulation of the rubber wasobserved.

Next, 3.9 mL of a 40 wt % antioxidant emulsion was added. The emulsionwas prepared by mixing 50 grams each of Santoflex 134 and TNPP(trisnonylphenyl phosphite) with 150 grams water and 10 grams of 40%K-oleate paste in water, adding aqueous NaOH to pH=10, followed bymixing in a Waring blender to form a stable emulsion. Using thisformulation, the addition of 0.052 grams of the emulsion per gram ofrubber provides 1 part per hundred of each stabilizer.

Next, 2500 mL of a 5% aqueous solution of Al₂(SO₄)₃ (“the coagulantsolution”) was placed in a 1-gallon Waring blender.

The mixture of [aqueous natural rubber latex+carbon black+antioxidantemulsion] was slowly poured into the coagulant solution, with theblender set to “low” speed, for 10 seconds. Then, a cover was placed onthe blender and the speed was raised to “medium” for 15 additionalseconds.

The resultant coagulant mixture was poured onto a No. 70 standard USsieve tray and rinsed with tap water. Some unincorporated carbon blackwas present in the rinse.

The coagulated rubber was placed in a 1 gallon glass jar and mixed withapproximately 3 quarts of tap water, followed by decanting the water,and repeated twice. Little or no free carbon black was observed in thelast rinse. The coagulant rubber was not squeezed so as to maintainmaximum porosity.

Next, the wet coagulated rubber was torn into approximately 1″ piecesand placed onto a Soxhlet extraction apparatus with a Whatman 90×200 mmcellulose extraction thimble, condenser, and 3000 mL flask. Then, 1000mL acetone was added to the flask and heated via an electric heatingmantle to reflux. The extraction was allowed to continue for 21 hours.The polymer remained quite porous and easily broke into discrete pieces.

Next, an acetone solution containing 1 part per hundred each ofSantoflex 134 and TNPP (sufficient to wet the coagulated rubber) wasadded and kneaded to force the solution into the pores of the coagulatedrubber.

The coagulated rubber was then placed into a vacuum oven at 50° C. for4.25 hours to remove the remaining acetone. Then, the acetone-freecoagulated rubber was placed in a forced air oven at 83° C. overnight.The sample is indicated as Sample 5. The total weight of the dryresultant rubber and carbon black was 69.28 grams.

Thermogravimetric analysis showed that the rubber contained 11.03 partsper hundred parts rubber of carbon black and acetone extractionindicated the presence of 3.5 weight % acetone extractables (based onthe weight of the total product [rubber+carbon black+extractables]).Correcting for the added antioxidants, the mixture contained 1.54 wt %(3.5−1.96 wt %) acetone extractables.

The rubber was next compounded using the standard test recipe referencedabove, but with the carbon black addition corrected to compensate forthe carbon black content of Sample 5. In Table 4 below, the compoundedrubber is indicated as Sample 5A and Hevea natural rubber is identifiedas Hevea #1.

TABLE 4 Micro Dumbbell Tensile @23 C Sample 5A Hevea #1 Max Stress (MPA)29.9 31.5 100% (MPA) 2.9 3.3 200% (MPA) 8.8 10.2 300% (MPA) 16.8 19.6Brk Strain % 472 448.9 Toughness 60.3 61.6 Micro Dumbbell Tensile @100 CSample 5A Hevea #1 Max Stress (MPA) 22.1 21.8 100% (MPA) 2.2 2.6 200%(MPA) 5.3 6.5 300% (MPA) 9.4 11.7 Brk Strain % 714 513.6 Toughness 83.251.4

From the data in Table 4, it is observed that the rubber productobtained in Example 5 compares favorably in performance to commerciallyavailable Hevea natural rubber (Hevea #1).

Example 6

This example demonstrates that acetone, by itself, will coagulate anaqueous natural rubber latex.

A 77.6 gram sample of an aqueous natural rubber latex marketed by YulexCorp. and having about 50% total solids was added slowly, with goodmixing, to 250 grams acetone. The natural rubber in the latex coagulatedto form a solid mass.

It is expected that an extraction of the rubber mass is necessary tolower the extractables content to a sufficient level, followed by adrying step to remove the residual extraction solvent from the rubber.It is expected that the efficiency of the extraction can be improved bythe addition of carbon black.

Example 7

An 88.4 gram sample of an aqueous natural rubber latex marketed by YulexCorp. and having 55% solids was placed in aluminum containers to aheight of 1 cm, and frozen at −30° C. in a freezer for 30 minutes tocreate a frozen latex. The frozen latex was removed from the freezer andthen unloaded from the containers.

After the frozen latex was melted at room temperature, the coagulatedmass was separated from the water by filtration. Extra water was removedby applying 40 tons pressure on the coagulated material for severalseconds. The resulting rubber had a mass of 57.6 grams. Another 9.4grams of water was removed from the coagulated rubber by steam drumdryer, which produced 48.2 grams of water-free coagulated rubber.

A 31.2 gram sample of the water-free coagulated rubber was extracted bythe Soxhlet procedure with acetone for 6 hours and found to contain12.7% acetone extractables.

The final weight of the resultant dried rubber was found to be 27.17grams. Although the amount of extractables remaining in the 27.17 gramsample of dried rubber itself was not measured, it is believed that theamount of extractables remaining in the sample was less than 2%. In thisexample, the acetone utilized in the Soxhlet procedure served as theextraction solution.

The resultant dried rubber (i.e., the 27.17 grams) was dissolved inhexane to form a uniform solution. One part per hundred rubber (byweight) of Santoflex 134 antioxidant was added and kneaded into thesolution. The hexane was then removed by vacuum drying.

The resultant dried and stabilized rubber was then placed in a 60° C.forced air oven for storage stability evaluation. Mooney viscosities asshown in Table 5 below were measured before and after aging in the oven.

TABLE 5 Before Aging After 7 Days ML (1 + 4) 88.3 83.4 T × 80 26.4825.84

From the data in Table 5, it is observed that the rubber productobtained in Example 7 compares favorably in properties to the rubberproduct obtained by other processes disclosed herein.

Example 8

A 2.2 gram sample of N339P carbon black, marketed by Columbian Chemicalsof Proctor, W. Va. was added to 100.73 grams of an aqueous naturalrubber latex marketed by Yulex Corp. and having about 51.1% total solidsfrom guayule shrub. The mixture was placed on a roller for 4 hours toachieve uniform mixing. The mixture was then placed in aluminumcontainers to a height of 0.5 cm, and frozen at −30° C. in a freezer for30 minutes to create a frozen mixture. The frozen mixture was removedfrom the freezer and then unloaded from the container.

After thawing at room temperature, the coagulated mass was separatedfrom the water by filtration. Extra water was removed by applying 40tons pressure on the coagulated material for several seconds, followedby vacuum drying.

The dried coagulated material was extracted by cutting into small piecesand soaked in 1.5 liters of acetone with strong agitation for 2 hours.After decanting the acetone, the resultant rubber was vacuum dried. Thefinal weight of the dry resultant rubber (with carbon black), indicatedhere as Sample 8, was found to be 53.66 grams.

A sample of 1.32 grams of the dry resultant rubber was extracted by theSoxhlet procedure using acetone and found to contain 1.52% acetoneextractables.

A 50 gram portion of the dry resultant rubber was dissolved in hexane toform a uniform solution. One part per hundred rubber (by weight) ofSantoflex 134 antioxidant was added and kneaded into the solution. Thesolvent was then removed by vacuum drying.

The resultant dry and stabilized rubber was further evaluated bycompounding it based on the standard natural rubber recipe as describedabove, with the exception that only 48 parts N339 carbon black wereadded per 100 parts rubber to account for the 2 parts carbon blackpreviously added. The dried rubber product is identified in Table 6,below, as Sample 8A, and compared to commercially available Hevearubber, identified as Hevea #1.

TABLE 6 Micro Dumbbell Tensile @23 C Sample 8A Hevea #1 Max Stress (MPA)29.8 31.5 100% (MPA) 3.4 3.3 200% (MPA) 10.0 10.2 300% (MPA) 17.8 19.6Brk Strain % 472 448.9 Toughness 63.2 61.6 Micro Dumbbell Tensile @100 CSample 8A Hevea #1 Max Stress (MPA) 15.3 21.8 100% (MPA) 2.4 2.6 200%(MPA) 5.9 6.5 300% (MPA) 10.1 11.7 Brk Strain % 565 513.6 Toughness 50.351.4

From the data in Table 6, it is observed that the rubber productobtained in Example 8 compares very favorably in performance tocommercially available Hevea natural rubber.

To the extent that the term “includes” or “including” is used in thespecification or the claims, it is intended to be inclusive in a mannersimilar to the term “comprising” as that term is interpreted whenemployed as a transitional word in a claim. Furthermore, to the extentthat the term “or” is employed (e.g., A or B) it is intended to mean “Aor B or both.” When the applicants intend to indicate “only A or B butnot both” then the term “only A or B but not both” will be employed.Thus, use of the term “or” herein is the inclusive, and not theexclusive use. See Bryan A. Garner, A Dictionary of Modern Legal Usage624 (2d. Ed. 1995). Also, to the extent that the terms “in” or “into”are used in the specification or the claims, it is intended toadditionally mean “on” or “onto.” Furthermore, to the extent the term“connect” is used in the specification or claims, it is intended to meannot only “directly connected to,” but also “indirectly connected to”such as connected through another component or components.

While the present application has been illustrated by the description ofembodiments thereof, and while the embodiments have been described inconsiderable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. Therefore, the application, in its broaderaspects, is not limited to the specific details, the representativeapparatus, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of the applicant's general inventive concept.

We claim:
 1. A process for recovering rubber from an aqueous naturalrubber latex that contains natural rubber, water, extractables, and oneor more additives, and is essentially free of lignocellulosic plantmaterial, comprising the steps of: contacting the latex with aseparating solvent composition comprising at least one hydrocarbonsolvent and at least one organic polar solvent whereby at least 75% ofthe natural rubber is separated from the water and a solution having aviscous rubber phase containing natural rubber and a reduced amount ofextractables is formed; isolating the viscous rubber phase; and dryingthe viscous rubber phase to create a resultant natural rubber whereinthe amount of extractables is 5% by weight or less.
 2. A process asclaimed in claim 1 further comprising, prior to drying the viscousrubber phase, the step of contacting the viscous rubber phase with atleast one extracting solvent composition, wherein said at least oneextracting solvent composition comprises at least one organic polarsolvent.
 3. A process as claimed in claim 1 wherein said at least onehydrocarbon solvent is present in an amount between 5% and 60% by weightand said at least one organic polar solvent is present in an amountbetween 95% and 40% by weight, based on the total weight of saidseparating solvent composition.
 4. A process as claimed in claim 2wherein the amount of extractables in the resultant natural rubber is 1%by weight or less.
 5. A process as claimed in claim 4 wherein said atleast one extracting solvent composition comprises acetone.
 6. A processas claimed in claim 2 wherein, in said separating solvent composition,said at least one hydrocarbon solvent is present in an amount between 5%and 25% by weight and said at least one organic polar solvent is presentin an amount between 75% and 95% by weight, said at least one extractingsolvent composition comprises acetone, and the amount of extractables inthe resultant natural rubber is 1% by weight or less.
 7. A process forrecovering rubber from an aqueous natural rubber latex that containsnatural rubber, water, extractables, and one or more additives, and isessentially free of lignocellulosic plant material, comprising the stepsof: contacting the latex with a first treatment solution whereby atleast 75% of the natural rubber coagulates and a mixture containingcoagulated natural rubber, water, and extractables is formed; collectingthe coagulated natural rubber; contacting the coagulated natural rubberwith a second treatment solution comprising at least one organic polarsolvent in order to preferentially solvate the extractables; and dryingthe coagulated natural rubber to create a resultant natural rubberwherein the amount of extractables is 5% by weight or less.
 8. A processas claimed in claim 7 wherein the first treatment solution comprises anorganic acid, a mineral acid, or a salt thereof
 9. A process as claimedin claim 7 wherein the first treatment solution comprises at least oneorganic polar solvent, said first and second treatment solutions beingthe same or different.
 10. A process as claimed in claim 8 wherein thefirst treatment solution comprises an acid selected from the groupconsisting of acetic acid, formic acid, sulfuric acid, aqueous aluminumsulfate, aqueous calcium chloride, and mixtures thereof
 11. A process asclaimed in claim 9 wherein said at least one organic polar solvent inthe first treatment solution is selected from the group consisting ofalcohols having from 1 to 8 carbon atoms and ketones having from 3 to 8carbon atoms, and wherein said at least one organic polar solvent in thesecond treatment solution is selected from the group consisting ofalcohols having from 1 to 8 carbon atoms and ketones having from 3 to 8carbon atoms.
 12. A process as claimed in claim 7 further comprising,prior to contacting the latex with a first treatment solution, the stepof adding carbon black to the latex in an amount between 0.5% and 6% byweight of the resultant dried natural rubber.
 13. A process as claimedin claim 7 further comprising, prior to contacting the coagulatednatural rubber with a second treatment solution, the step of addingcarbon black to the coagulated natural rubber in an amount between 0.5%and 6% by weight of the resultant dried natural rubber.
 14. A process asclaimed in claim 12 wherein the first and second treatment solutionsconsist essentially of acetone.
 15. A process as claimed in claim 13wherein the first and second treatment solution consist essentially ofacetone.
 16. A process as claimed in claim 8 wherein the amount ofextractables in the resultant natural rubber is 2% by weight or less.17. A process as claimed in claim 9 wherein the amount of extractablesin the resultant natural rubber is 2% by weight or less.
 18. A processfor recovering rubber from an aqueous natural rubber latex that containsnatural rubber, water, extractables, and one or more additives, and isessentially free of lignocellulosic plant material, comprising the stepsof: freezing the latex, whereby at least 75% of the natural rubbercoagulates and a mixture containing coagulated natural rubber, water,and extractables is formed; thawing the mixture; collecting thecoagulated natural rubber; contacting the coagulated natural rubber withan extraction solution comprising at least one organic polar solvent inorder to preferentially solvate the extractables; and drying thecoagulated natural rubber to create a resultant natural rubber whereinthe amount of extractables is 5% by weight or less.
 19. A process asclaimed in claim 18 further comprising, prior to freezing the latex, thestep of adding carbon black to the latex in an amount between 0.5% and6% by weight of the dried natural rubber.
 20. A process as claimed inclaim 18 further comprising, prior to contacting the coagulated naturalrubber with an extraction solution, the step of adding carbon black tothe coagulated natural rubber in an amount between 0.5% and 6% by weightof the resultant dried natural rubber.